Thermal Interface Materials (TIMs) are thermally conductive materials typically designed to act as a thermal interface between a heat generating component, e.g. an electrical component, and a heat conducting element, e.g. a heat sink or a heat spreader, and to fill any voids or irregularities that may exist in the mating surfaces of the heat generating component and the heat conducting element. The latter increases the contact area between the heat generating component and the heat sink, thereby effectively reducing the thermal impedance between them and allowing for the efficient transfer of heat. To be effective, TIMs preferably have very low bulk thermal resistivity and a viscosity low enough to allow them to flow away from points of contact towards any voids that may exist between the mating surfaces. The TIM must also stay in place once it flows, and remain flexible during temperature changes. If the viscosity is too low, the TIM may flow out from between the mating surfaces leaving voids and resulting in higher thermal impedance.
There are a number of TIMs used on the market, like thermal paste, or grease, phase changing materials, thermal pads etc. The most important function of a TIM is to guarantee maximized physical contact between two interfaces, i.e. the mating surfaces of the heat generating component and the heat conductive element. Only when this function is fulfilled, the bulk conductivity of the TIM comes into play. Without proper physical contact with both interfaces, the bulk conductivity has a minor influence on the thermal resistance. It is for this reason that thermal paste or grease is usually a best performer thermally, despite its (in general) relatively low bulk conductivity: paste is structured in a way that it has extremely good wetting capabilities and thus establishes maximal physical contact between the interfaces. Phase change materials, PCMs, are engineered to show similar behavior. Also in the design of thermal pad materials, great care is taken to allow for maximal physical contact. The main driver behind the use of thermal gels and gap fillers is also based on maximal physical contact with both interfaces. For all these TIMs there is a certain contact pressure dependence of the thermal performance of the interface, which is generally strongest for the thermal pad materials: the higher the pressure, the better the physical contact (larger total contact area). Finally, there are thermal glues around that overcome the above mentioned issues but also have specific disadvantages of themselves, as will be described in the next section.
As the TIM generally requires a minimum contact pressure to ensure physical contact and thus optimal thermal performance, this complicates system designs in the sense that the system has to guarantee such a minimum pressure over lifetime. The before mentioned contact pressure requirement for optimal thermal performance has a number of disadvantages. First of all, a system that makes use of a TIM must be designed carefully with respect to the specified or required minimum contact pressure between the interfaces that hold the TIM. Features to realize the contact pressure can be screws, clamps, etc. which increases the cost of the system. Further, mounting with a predetermined pressure on the TIM surfaces is difficult, especially when utilizing plastic components or components having a certain tolerance. The externally applied pressure must be applied carefully. Too much geometric non-uniformity in the applied pressure can cause the interfaces to change shape (e.g. warp, curl, etc.). With that comes a fair risk that physical contact between the TIM and both interfaces is (partially) lost, and thus such an applied contact pressure is counter-acting the thermal performance, since establishing physical contact is the most important function of a TIM as explained above. Additionally, the contact pressure itself can cause the TIM to pump-out under thermal cycling by switching the device on and off (normal usage). This effect has been observed many times, especially in case of thermal pastes and PCMs, where the TIM subsequently contaminates other critical parts of the system. Furthermore, a TIMs thickness and hardness should be chosen carefully with respect to the properties or characteristics of both interfacial surfaces, such as roughness and curvature. This means that the selected TIM should be able to overcome the specified surface properties. Finally, thermal glue generally overcomes all these issues. However, glue has a huge disadvantage of itself, being risks of cracking and delaminating under thermal cycling induced by (often inherent) Coefficient of Thermal Expansion (CTE) mismatches, especially for larger surface areas. Moreover, glue always needs some kind of curing process which complicates the assembly process and increases production cost significantly.